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Aromatase inhibitors

Aromatase inhibitors are drugs that lower estrogen levels by stopping an enzyme in fat tissue (called aromatase) from changing other hormones into estrogen. These drugs don’t stop the ovaries from making estrogen. Since most estrogen in the body is made by the ovaries, these drugs only lower estrogen levels in women whose ovaries aren’t making estrogen (such as those who have already gone through menopause). The drugs in this class include:

These drugs are used mainly to treat hormone receptor-positive breast cancer. They are being studied to see if they can lower breast cancer risk, but so far they are not yet approved for this use in the US.

These drugs are taken once a day as pills.

Can aromatase inhibitors lower the risk of breast cancer?

Studies have shown that both exemestane and anastrozole can lower the risk of breast cancer in post-menopausal women who are at increased risk of the disease.

In one study, taking exemestane for 3 years lowered the risk of breast cancer overall (invasive cancer plus ductal carcinoma in situ) by about half (47%).

In another study, taking anastrozole for 5 years lowered the risk of breast cancer overall by about half (47%).

A study to see if letrozole can lower breast cancer risk is going on now.

What are the risks and side effects of aromatase inhibitors?

The most common side effects of aromatase inhibitors are symptoms of menopause, such as hot flashes, night sweats, and vaginal dryness. These drugs can also cause muscle and joint pain. This side effect can be serious enough to cause some women to stop taking the drugs.

Unlike tamoxifen and raloxifene, aromatase inhibitors tend to speed up bone thinning, which can lead to osteoporosis. People with osteoporosis can break bones with little trauma.

Aromatase inhibitors do not seem to increase the risk of serious blood clots or cancer of the uterus, like tamoxifen and raloxifene do.

Aromatase inhibitors to reduce breast cancer risk: More research is needed

Aromatase inhibitors may someday prove to be as good as or even better than tamoxifen or raloxifene in reducing breast cancer risk, but they haven’t been well studied for this use. More study is needed to see who would most benefit and how long treatment should be continued.